The invention relates to a trackball that is used notably for pointing at objects on a screen of a computer system.
A trackball is understood to be an assembly having a fixed body and a movable part such as a sphere, for example. The fixed part comprises sensors for providing information that is representative of the position of the sphere with respect to the fixed part. The information may be associated with a position on the screen by means of a computer of the piece of equipment. An operator moves the sphere with his fingers in order to reach a desired position on the screen and thus to point at an object displayed on the screen.
The invention is particularly useful for a trackball belonging to an item of equipment installed on board an aircraft. The trackball can thus be used to select flight parameters that are displayed on the screen of the system. More generally, the trackball makes it possible to point at objects displayed on the screen.
The aircraft can be subjected to turbulence which is likely to make it more difficult to point at objects. More precisely, the turbulence generates vibrations, which can result in uncontrolled movements of the sphere. Even in the absence of turbulence as such, minor acceleration phenomena to which the equipment is subjected prevent members of crew from pointing at a graphical zone beyond a certain degree of precision.
A conventional solution to this problem consists in oversizing the objects which are selectable on the screen so as to take this limitation into account. As a result, this reduces the number of objects that can be displayed on a given display surface.
Attempts have also been made to stabilize the operator's hand by creating suitable support surfaces. One example is given in the patent application published under the number EP 1 552 376, where the fixed part of the trackball has an ergonomic shape intended to take the operator's palm, hence its name of “palm rest”. However, such a palm rest is unable to completely stabilize the sphere through the operator's fingers.
These two solutions are often associated with means for braking the sphere. These means make it possible to limit the uncontrolled movements of the sphere in the event of vibrations.
The brake is of course secured to the fixed part of the trackball and exerts a force on the sphere for example by means of a pad that rubs against the sphere. During movements of the sphere, the pressing of the pad generates a friction torque which opposes rotations of the sphere. The pad is kept pressed against the sphere by means of a spring that bears against the fixed part.
Assuming that the sphere is accessible to the operator's fingers from above, one solution that has already been implemented consists in positioning a friction surface on the ball above the plane of symmetry of the sphere. The friction surface is generally realized by a membrane made of plastics material. The force exerted along the vertical axis of the sphere by the friction surface is realized by an elastomer or a spring system that applies pressure to the friction surface.
This arrangement brings about a number of problems. The area that is usable by the operator is reduced on account of the presence of the friction surface above the plane of symmetry of the sphere.
The friction torque generated by the brake is highly dependent on the precision of coaxiality between the vertical axis of the sphere and the axis of the friction surface. The sphere is generally positioned on pivots that form sensors for providing information relating to the position of the sphere with respect to the fixed part. The deviation between the axes of the sphere and of the brake depends on a chain of dimensions that passes through numerous mechanical parts. The precision of coaxiality requires precise assembly and machining operations.
The friction torque generated depends on the pressure exerted by the elastic device, this pressure depending itself on the vertical position of the sphere. In order to limit this dependence, provision can be made of vertical wedging means of the brake. This solution is expensive since the wedging means need to be adjusted jointly for each trackball.
The torque generated also varies greatly depending on the variations in diameter of the sphere, in particular during variations in temperature, causing said sphere to rise against the braking device when it is used at high temperature or to lower when it is used at low temperature.
Moreover, the friction torque is set when the trackball is assembled. In order to modify this setting, it is necessary to remove the brake, this being difficult to accept in the aeronautical field.
Finally, it has been found that the brake is particularly delicate when used outside its normal use position, such as if it is turned over during transport.